Interpretive Summary: The fungus, Fusarium oxysporum f. sp. vasinfectum, causes the Fusarium wilt disease in cotton. This disease can result in severe yield losses and even plant death. However, the extent of injury depends on cotton cultivar and the race or type of Fusarium. Types of Fusarium can also be distinguished chemically based on specific DNA sequences (molecular markers). To better understand the diversity of Fusarium in the cotton growing regions of the southeastern U.S., samples of Fusarium were collected from diseased plants in four states. The samples were compared based on molecular markers and their respective abilities to cause disease in six cotton cultivars. These analyses identified six Fusarium types. Two of the Fusarium types were known from previous collections, but four types were not previously known from the Southeast. Three of the new Fusarium types caused disease symptoms in several of the cotton cultivars, although none severely injured the Pima variety Phytogen 800. These results suggest the potential for increased problems with Fusarium wilt in cotton of the Southeast, and indicate the need for a more extensive survey of Fusarium types.

Technical Abstract:
Sixty-one isolates of Fusarium oxysporum f. sp. vasinfectum were collected from cotton plants (Gossypium spp.) with symptoms of Fusarium wilt to determine the composition of races present in the southeastern U.S. Analysis of partial sequences of the translation elongation factor gene revealed four novel genotypes, as well as the presence of races 3 and 8 for the first time in the U.S. outside of California. The majority of isolates (16 of 27) sampled from Arkansas were novel genotypes. A subset of isolates representing the novel genotypes was compared with previously described races using sequences from translation elongation factor, phosphate permase, and beta tubulin genes, and their pathogenicity on a total of six Upland (G. hirsutum L.) and Pima (G. barbadense L.) cotton cultivars. Two of the novel genotypes belonged to a clade containing races 1, 2, 4, 6, and 8 and two shared ancestry with race 3. All new genotypes were pathogenic to at least some of the cotton cultivars tested. The Pima cultivar Phytogen 800 was relatively resistant to all genotypes of the pathogen. These results indicate that the population of F. oxysporum f. sp. vasinfectum in the southeastern U.S. is more diverse than previously recognized.